CS 501: Software Engineering

1. CS 501: Software Engineering Lecture 14 System Architecture and Design 2

2. Administration

3. Architectural Styles An architectural style is system architecture that recurs in many different applications. See: Mary Shaw and David Garlan, Software architecture: perspectives on an emerging discipline. Prentice Hall, 1996

4. Architectural Style: Pipe Example: A three-pass compiler Lexical analysis Code generation Parser Output from one subsystem is the input to the next.

5. Mailing and reports Customer services Data input and validation Architectural Style: Master File Update Example: billing system for electric utility Master file update Advantages: Efficient way to process batches of transactions. Disadvantages: Information in master file is not updated immediately.

6. Architectural Style: Repository Example: A digital library Input components Transactions Repository Advantages: Flexible architecture for data-intensive systems. Disadvantages: Difficult to modify repository since all other components are coupled to it.

7. Architectural Style: Repository with Storage Access Layer Repository Input components Storage Access Transactions This is sometimes called a "glue" layer Data Store Advantages: Data Store subsystem can be changed without modifying any component except the Storage Access.

8. Data Intensive Systems: Merger of Two Banks Each bank has a database with its customer accounts. The databases are used by staff at many branches and for back-office processing. These systems are examples of Repository Architectural Style. The requirement is to integrate the two banks so that they appear to the customers to be a single organization and to provide integrated service from all branches. This is an example of working with legacy systems.

9. Merger of Two Banks: Options A B ??? ???

10. Merger of Two Banks: Architectural Options I. Convert everything to System A convert databases retrain staff enhance System A (software and hardware) discard System B II. Build an interface between the databases in System A and System B III. Extend client software so that it can interact with either System A or System B database

11. Merger of Two Banks: Interface between the Databases Bank A Bank B Teller transactions Teller transactions Data exchange API Data trans-form Data trans-form Accounts database Accounts database Problem Accounts databases are rarely exactly equivalent. Batch input Batch input

12. Data Intensive Systems: Distributed Data Distributed Data Data is held on several computer systems. A transaction may need to assemble data from several sources.

13. Data Intensive Systems: Replication Replication Several copies of the data are held in different locations. Mirror: Complete data set is replicated Cache: Dynamic set of data is replicated (e.g., most recently used) With replicated data, the biggest problems are concurrency and consistency.

14. Architectures for Distributed Computing An application that is running on one computer wishes to use data or services provided by another: • Network connection private, public, or virtual private network location of firewalls • Protocols point-to-point, multicast, broadcast message passing, RPC, distributed objects stateful or stateless • Performance quality of service

15. Network Choices Public Internet: Ubiquitous -- worldwide Low cost Private network: Security / reliability Predictable performance Choice of protocols (not constrained to TCP/IP)

16. Network Choices:Quality of Network Services Criteria in choosing a system architecture Performance Maximum throughput Latency Variations in throughput Real-time media (e.g., audio) Business Suppliers, cost Trouble shooting and maintenance

17. Private network Public network Firewall Network Choices: Firewall A firewall is a computer at the junction of two network segments that: • Inspects every packet that attempts to cross the boundary • Rejects any packet that does not satisfy certain criteria, e.g., an incoming request to open a TCP connection an unknown packet type Firewalls provide security at a loss of flexibility and a cost of system administration.

18. Architectural Style: Client/Server Example: the Web Apache server Firefox client The control flows in the client and the server are independent. communication between client and server follows a protocol. In a peer-to-peer architecture, the same component acts as both a client and a server.

19. Distributed Computing: Multicast Broadcast Searching User User interface service Databases This is an example of a multicast protocol. The primary difficulty is to avoid troubles at one site degrading the entire system (e.g., every transaction cannot wait for a system to time out).

20. Distributed Computing:Stateless Protocol v. Stateful Stateless protocol Example: http Open connection Send message Return reply Close connection State in http must be sent with every message (e.g., as parameter string) Cookies are a primitive way of retaining some state

21. Distributed Computing:Stateless Protocol v. Stateful Stateful (session) protocol Example: Z39.50 Open connection Begin session Interactive session End session Close connection Client and server remember the results of previous transactions (e.g., authentication, partial results) until session is closed.

22. Distributed Computing: Distributed Caches The Domain Name System First attempt to resolve www.cs.cornell.edu .edu server 1 cornell.edu server 2 3 cs.cornell.edu server

23. Distributed Computing:Distributed Caches The Domain Name System Better method local DNS server .edu server 1 2 cornell.edu server almaden.ibm.com cornell.edu ece.cmu.edu ibm.com acm.org .edu 3 Local cache cs.cornell.edu server

24. Distributed Computing:Distributed Caches The Domain Name System For details of the actual protocol read: Paul Mockapetris, "Domain Names - Implementation and Specification". IETF Network Working Group, Request for Comments: 1035, November 1987. http://www.ietf.org/rfc/rfc1035.txt?number=1035

25. Distributed Computing: Intermittent Connectivity This is an example of an epidemic protocol. Such protocols are especially useful in networks with intermittent connectivity, e.g., mobile computing. The biggest problem is ensuring that the data is distributed effectively. Example: Usenet

26. Time-Critical Systems A real time (time-critical) system is a software system whose correct functioning depends upon the results produced and the time at which they are produced. • A soft real time system is degraded if the results are not produced within required time constraints • A hard real time system fails if the results are not produced within required time constraints

27. Time-Critical System: Routers and Other Network Computing • Interoperation with third party devices => remote devices may have faulty software • Restart after total failure • Defensive programming -- must survive => erroneous or malicious messages => extreme loads => time outs, dropped packets, etc. • Evolution of network systems => Support for several versions of protocols

28. Architectural Style: Time Critical System - Daemon Spawned process Daemon Example: Web server The daemon listens at port 80 When a message arrives it: spawns a processes to handle the message returns to listening at port 80

29. Time-Critical System: Software Development • Developers of advanced time-critical software spend almost all their effort developing the software environment: • Monitoring and testing -- debuggers • Crash restart -- component and system-wide • Downloading and updating • Hardware troubleshooting and reconfiguration • etc., etc., etc.

30. Software Considerations of System Architectures: Performance Resource considerations may dictate software design and implementation: • Low level language (e.g., C) where programmer has close link to machine • Inter-process communication may be too slow (e.g., C fork). • May implement special buffering, etc., to control timings

31. Time-Critical System: Buffering a CD Controller for Automobile 3 4 1 2 Input block 5 Output block 6 7 Circular buffer

32. Time-Critical System: Autonomous Land Vehicle GPS Steer Sonar Model Control signals Throttle Laser Controls Sensors Signal processing

33. Architectural Style:Model/Controller/View Example: An unmanned aircraft Model View Controller Controller: Sends control signals to the aircraft and receives instrument readings. Model: Translates data received from and sent to the aircraft into a model of flight performance. It uses domain knowledge about the aircraft and flight. View: Displays information about the aircraft to the user.

34. Software Considerations of System Architectures In some types of system architecture, non-functional requirements of the system may dictate the software design and development process.

35. Software Considerations of System Architectures: Multi-Threading Several similar threads operating concurrently: • Re-entrant code -- separation of pure code from data for each thread • May be real-time (e.g., telephone switch) or non-time critical The difficult of testing real-time, multi-threaded systems may determine the entire software architecture. • Division into components, each with its own acceptance test.

36. Time-Critical System:Embedded Real-time Systems Software and hardware are combined to provide an integrated unit, usually dedicated to a specific task: • Digital telephone • Automobile engine control • GPS • Scientific instruments • Seat bag controller The software may be embedded in the device in a manner that cannot be altered after manufacture.

37. Software Considerations:Embedded Real-time Systems Hardware v. Software Design of embedded systems requires close understanding of hardware characteristics • Special purpose hardware requires special tools and expertise. • Some functions may be implemented in either hardware of software (e.g., floating point unit) • Design requires separation of functions Distinction between hardware and software may be blurred.

38. Software Considerations of System Architectures: Continuous Operation Many systems must operate continuously • Software update while operating • Hardware monitoring and repair • Alternative power supplies, networks, etc. • Remote operation These functions must be designed into the fundamental architecture. *

39. Coupling and Cohesion Coupling is a measure of the dependencies between two subsystems. If two systems are strongly coupled, it is hard to modify one without modifying the other. Cohesion is a measure of dependencies within a subsystem. If a subsystem contains many closely related functions its cohesion is high. An ideal breakdown of a complex system into subsystems has low coupling between subsystems with high cohesion within subsystems. *